Known in the art are work apparatus that include motors for facilitating the performance of selected work operations on a work material with a tool. Examples of such work apparatus include, but are not limited to, plotters, printers, cutting apparatus, and machining apparatus. Such work apparatus can include a variety of motors, such as a motor or motors to provide relative movement between the tool and the work material. For example, the work apparatus can include a work head that mounts a xe2x80x9cZ axisxe2x80x9d motor for moving the tool toward and away from the work material for selectively engaging the work material with the tool. Other motors can be provided for moving the tool or the work material along X and Y axes, which are perpendicular to the Z axis. Provision can also be made for rotational movement about a selected axis or selected axes. The number of motors and movement axes of a particular work apparatus depends on the degrees of freedom of movement between the tool and the workpiece required to perform the desired work operation.
Control of the motors, and hence of the movement of the tool relative to the work material, is important to properly perform the work operations. Undesirable oscillation or other movement is to be avoided. Often, feedback signals are used in a servo loop to control one or more of the motors of the work apparatus. Typically, a position feedback signal is compared to an instruction signal to produce an error signal representative of the difference between the actual and desired positions of the motor or tool. In addition, a velocity sensor can be coupled to the motor to provide a velocity feedback signal responsive to the velocity of the motor. The utility feedback provides a xe2x80x9cviscousxe2x80x9d damping, akin to the damping provided to a shock absorber in an automobile, to help avoid unwanted movement of the tool.
Unfortunately, the velocity sensor does not always provide a suitable velocity feedback signal to fully compensate for certain undesirable movements or oscillations. In such a case, work operations can be detrimentally affected. For example, consider a cutting apparatus wherein the work material is a sheet material, such as a sheet of vinyl or paper, extending in the X-Y plane and having a thickness along the Z axis. In the sign making industry, such cutting apparatus cut graphic products from a colored vinyl that is releasably secured by adhesive to a backing material. The outline of the graphic product is cut by the tool, and the graphic product, such as the letter xe2x80x9cAxe2x80x9d shown in FIG. 1, is peeled from the releasable backing and secured, using the adhesive that remains on the vinyl, to a sign board, plate glass window, truck panel, etc., to fabricate all or part of a sign. It is desirable to cleanly cut the outline of the graphic product, preferably by cutting all the way through the vinyl, but not into the backing material. The work head typically includes a low friction linear motor for controlling the tool along the Z axis. The motor is typically operable in a force mode, wherein it is controlled such that the tool engages the work material with a selected force, as well as in a position mode, where it is controlled so as to move the tool to a selected position along the Z axis. Typically the motor includes the velocity sensor described above, and the motor is used in the force mode and instructed to apply a force to the vinyl sufficient to cut through the vinyl and not the backing. Unfortunately, small oscillations of the tool along the Z axis often occur when cutting and translating the tool, and these oscillations cause a stitching effect wherein portions of the vinyl are not cut all the way through. When peeling the vinyl from the releasable backing, the vinyl can tear, ruining the graphic product. These oscillations occur despite the use of the velocity sensor, which does provide adequate control for repositioning the tool.
As another example where control of the tool is important, work operations, such as the aforementioned cutting of vinyl, can involve rapid or repeated engagement and disengagement between the tool and the work material. When cutting the aforementioned graphic product, the tool can bounce when instructed to move to engage the work material, again, despite the use of the aforementioned velocity sensor in a feedback loop. Such bouncing can mar the vinyl, detrimentally affecting or ruining the vinyl.
Although methods and apparatus, such as the use of velocity feedback, are known for enhancing the control of motors, and hence of the relative movement between the tool and workpiece, unfortunately these known methods and apparatus can be undesirably complex, expensive, or, as noted above, can fail to provide adequate control in certain applications for addressing the above problems.
Accordingly, it is an object of the present invention to address one or more of the foregoing disadvantages or drawbacks of the prior art.
According to one aspect of the invention, the foregoing and other objects are addressed by providing a control circuit for controlling a motor responsive to feed back signals. The control circuit includes a velocity sensor for coupling with the motor for providing a velocity feedback signal responsive to the velocity of the motor, and circuitry for providing a second feedback signal responsive to the back emf of a coil associated with the motor.
In another aspect of the invention, the coil can be a drive coil of the motor, wherein a xe2x80x9cdrive coilxe2x80x9d refers to a coil that produces a magnetic field for moving the motor. In addition, the control circuit can include a filter for filtering the feedback signal. The filter can be a low pass, an all pass, or a high pass filter. The velocity sensor can include an optical encoder including an encoder element that includes a plurality of spaced indicia.
In yet an additional aspect of the invention, there is provided a control system having a motor and controls therefore, the controls including a velocity sensor for feeding back a velocity feedback signal responsive to the velocity of the motor, and further including a second feedback path for feeding back a feedback signal responsive to the back emf of a coil associated with the motor. The motor is controlled responsive to the feedback signal.
In a further aspect of the invention, the invention provides an apparatus for performing work operations with a tool on a work material responsive to an instruction signal and having improved control of the tool. The work apparatus can include a support structure having a support surface for supporting the work material, a work head mechanically coupled to the support structure and for mounting the tool, and at least one motor for moving the tool toward and away from the work material for selectively engaging the work material. The work apparatus can also include control circuitry for providing a control signal to the motor responsive to the instruction signal and selected feedback signals. The circuitry can include a velocity sensor for providing a velocity feedback signal responsive to the velocity of the motor and a summer for combining the velocity feedback signal with the instruction signal and with a feedback signal responsive to a voltage associated with the motor to provide the control signal.
The invention also include methods practiced in accordance with the teachings herein.